The present disclosure relates generally to an improved design for assembling a fuel-cell stack, and more particularly to a way to distribute an acceleration load over a fuel-cell stack to secure and maintain the relative position of the fuel cells within the stack after exposure to impacts and other high acceleration loads.
A significant benefit to using fuel cells to convert a fuel into usable electricity via electrochemical reaction is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel-cell stack.
The delivery of the reactants to the MEA—as well as the removal of the byproduct water and the delivery of the cell-generated electrical current to the load—is facilitated through stacked engagement of the MEA, a gas-permeable diffusion medium (also called a gas diffusion medium (GDM)) and a multi-channeled bipolar plate. In addition to establishing a planar facing relationship with the MEA and GDM, the bipolar plate defines a manifold as part of a frame-like structure that is sized to be placed about the periphery of the MEA and GDM to facilitate the reactant, coolant and byproduct movement within the stack.
Fuel-cell stacks placed within vehicles must be able to withstand severe load changes from acceleration and deceleration of the vehicle, as well as from crashes, accidents and related impacts. In particular, in order to continue to perform after exposure to high acceleration loads (for example, up to 160 g or more) during disruptive events such as a vehicle crash, the position of the fuel cells that make up the stack must be retained relative to one another. In such events, a high shearing force may cause sliding between adjacent cells of the stack (especially within the X-Z plane of the aforementioned Cartesian coordinate system). Small displacements between individual cells is magnified over the height of a large stack assembly (for example, a 100 micron cell shift can result in a 30 mm fuel-cell stackshift for a 300-cell fuel-cell stack assembly). Such problems may be exacerbated by cold start conditions where thermally-induced contraction may reduce the Y-axis compressive retention load that was placed on the cells during stack assembly, as well as by reduced inter-cell friction brought about by the use of surface treatments or inserts that may have low coefficient of friction attributes.
One way to avoid automotive fuel cell inter-plate or inter-cell shifting during these high-acceleration events is to leave datum pins that are used in stack assembly coupled to the stack even after the assembling process is complete; in this way, the pins provide additional resistance to the shearing movement between the adjacently-stacked plates or cells. In the present context, these shearing or in-plane shifts between adjacent cells or plates are premised on the understanding that the cell or plate stacking axis is orthogonal to the direction of travel of the vehicle being powered by such stack. As such, the stacking axis may be along a substantially vertical (i.e., Y) Cartesian axis so that the majority of inter-cell or inter-plate movement sought to be minimized is in the X-Z plane. It will be appreciated by those skilled in the art that the particular orientation of the cells, plates and stack isn't critical, but rather that the means used to avoid or reduce such inter-cell or inter-plate shifting are preferably arranged in an orientation that maximizes such avoidance. While the use of conventional datum pins and related structures are effective at maintaining the relative stacking alignment of the cells or plates when exposed to a high acceleration in-plane load, they can significantly add to the cost of assembly of the stack. Their continued presence within the stack also militates against disassembly in the event one of the cells or other stack components needs to be removed for service.
Another way to avoid automotive fuel cell inter-plate or inter-cell shifting during such a disruptive event is through the use of adhesives or supplemental support structure that can be formed between a housing wall and the stack. An example of this may be found in U.S. patent application Ser. No. 13/803,098 that was filed on Mar. 14, 2013 and entitled CELL RETENTION DESIGN AND PROCESS that is owned by the Assignee of the present application and incorporated herein by reference in its entirety; the approach taught therein uses an insertable adhesive-like potting compound between the lateral edges of the stacked plates and a rigid housing or related enclosure. Nevertheless, this approach is only applied after the cells and plates have been aligned and stacked, and therefore does nothing to help with the alignment of the cells and plates during the stacking process. Moreover, the permanent nature of the compound being used is not conducive to subsequent stack disassembly for repair or diagnostic analysis.
Yet another approach involves welding (or otherwise attaching) a tab that projects laterally from one or more of the edges of the generally rectangular bipolar plate. These tabs may be made to engage with one another along the through-the-thickness (i.e., Y-axis) dimension such that the tendency of each cell or plate within the stack to move in response to a shearing (i.e., in-plane) force is resisted by the interfering contact of the tab and cutout. While effective at preventing inter-cell/inter-plate movement, each tab must be individually joined to its corresponding plate.